Morphogenetic ovarian modules for oocyte production from germline stem cells

Oocyte production from early progenitors in the fetal ovary determines the ovarian reserve and impacts lifelong female reproduction, yet its developmental regulation remains elusive. Early progenitors or germline stem cells (GSCs) are not maintained in adult mammalian ovaries, and in mice they are lost closely following embryonic sex determination. Fundamental knowledge of vertebrate ovarian GSCs and how they are regulated to produce oocytes in the developmental context of the ovary is lacking. Here, we uncover a previously unrecognized ovarian developmental microenvironment, the ovarian module, in developing zebrafish and human ovaries. We show zebrafish ovarian modules comprise the morphogenetic unit of oocyte production, harboring GSCs at the young tip, which generate oocyte precursors and early differentiating oocytes in a spatially linear manner. We define module morphology, resolve its developmental dynamics, recapitulate its single-cell transcriptomic architecture, and identify Notch signaling as a regulator of GSCs. We map Notch pathway components in modules, identifying GSC as sender cells and their progeny as receiver cells, and functionally demonstrate that Notch activity is required to suppress the GSC fate and to promote differentiation. Ovarian modules persist in adult zebrafish ovaries, highlighting a similar framework in oocyte production during homeostasis. We further demonstrate that modules are conserved in human fetal ovaries, where POU5F1/OCT4+ germ cells and differentiating oocytes are similarly organized, and Notch components are expressed in equivalent cell types, supporting the module as a fundamental strategy in oocyte production. Our findings propose new developmental principles for oogenesis and establish new paradigms in female GSC biology and for women’s reproductive regenerative medicine.